DE2138942B2 - Acoustic-optical filter - Google Patents

Description

4. Filter according to claim 3, characterized marked 25 crystal set to generate an acoustic wave, that the medium (2) gene a crystal plate, the inside of a surface of the crystal longitudinally (2) which is reflected laterally to the opposite surface due to a corresponding dimensioning, its thickness is designed as a thickness oscillator. This electro-acoustic converter is not a reso-

5. Filter according to claim 4, characterized in that it is nanzvorrichtung and therefore requires a larger one draws that the thickness of the crystal plate (2) for 30 amount of acoustic energy to a predetermined a resonant excitation in the harmonic range to produce diffraction of the filtered light. A coarse is. like acoustic-optical filter is in the essay

6. Filter according to claim 5, characterized in "Acousto-Optic Tunable Filter" in the "Journal records that the frequency of the RF signal of the Optical Society of America «. Vol. 59. 1969, step by step to stimulate various discrete 35 pp. 744 to 747th described.

Harmonics in the crystal plate (2) changeable The object of the present invention is to reasons, an acousto-optical filter of the entrance

7. Filter according to one of claims 1 to 6 mentioned genus in such a way that the characterized in that the medium (2) constructive effort for generating the akuein Polarizer (12; 22) is connected upstream. 40 stic wave is as low as possible.

This object is achieved according to the invention in that the optically anisotropic medium is piezoelectric

has tric properties and is associated with

an electrode arrangement that can be acted upon by the HF signal forms the electro-acoustic transducer. Advantageous developments or further training

The invention relates to an acousto-optical system. The invention is defined by the subclaims marked with a in the direction of a downstream.

Translucent, optically anisotropic analyzer, the following are preferred embodiments

medium in which by means of an electro-acoustic. 50 play the invention with reference to the drawings a transducer fed by an HF signal acoustically: it represents

Waves collinear with the direction of the light beam- F i g. 1 is a line block diagram of an acoustic

Vt'eges can be generated by the medium. optical light filter.

So far, acoustic resonators have been identified as acu- F i g. 2 a graph of the percentage of transmis-

Table optical element used by light filters. 55 power at the optical bandpass frequency Light of a first polarization direction was collided by the filter over the acoustic power density in near by a high-frequency acoustic wave in the acousto-optical element for filtered light a birefringent crystal diffracted to the same polarization as the input light and Light beam from the first direction of polarization in for filtered light of a cross polarization in clubs to rotate the second polarization direction. The relationship to the entrance light.

Output light was then applied to the polarization. 3 is a line block diagram of another

lion analyzes only light of the second Polari embodiment of an acousto-optical light to let sationrichlung through. The light of the two-filters,

len direction of polarization had a frequency which F i g. Figure 4 is a line block diagram of another

depending on the frequency fluctuations of the high embodiment of an acousto-optical filter The frequency of the acoustic wave in the crystal changed FIG. 5 is a line diagram of a ring laser.

that was. Such acousto-optical filters were Fig. 6, a line diagram of a tunable

Tunable from 5000 to 7500 A by using the Fre- Lasers.

7 shows a diagram of the gain over the A ₀ - k _e = k _a

optical path length _; for the laser of FIG. 12. is fulfilled, whereby the indices ₀ , _e and _{a are} the usual

In Fig. 1 is an acoustic-optical light filter 1 ¹ ^ " ^U * ^d ^ T ^ u * ° ^ ^chsn * ' ^dl f " ^nd ^

shown. This filter 1 is similar to what 5 ^{denotes acoustic wave} . It does if

in the mentioned article »Acousto-Optic Tunable J ^e % ****" and acoustic frequencies / "and;"

Filters «in the journal» Journal of the Optical satisfy the equation:

Society of America ”; this is true with, _ __ £ fa _G jq \

with the exception that the optically anisotropic medium * ° ~ ~~ \ F ~. \ n ~

is also piezoelectric and the acoustic wave in io

the optically anisotropic medium by an electrical wherein ~ the ratio of optical velocity

S ^keit r ^v _M ^k r ^zu r ^aku r ϊτ tfs

table anisotropic piezoelectric plate 2 overall ^{m d} ™ ^{Medl "m" nd} ^ ^{n. dl?} birefringence of

suitable piezoelectric optically anisotropic media » ₅ £" *? "%; f '? ¹ *? ^ ** ¹¹ * ^ ¹ Jf "" ^! LΓ £ S

exhibit piezoelectric birefringent crystals like the pass band of the optical riter is through the

Quartz or LiNbO ₃ . In addition to crystal 2, given by ^equation :

opposite sides of the crystal are optically 1 _:.

transparent electrodes 3 and 4 arranged. Appropriate ^B · ^W · "^ - '. -, ~ ^cm "' ^

nete optically transparent electrodes 3 and 4 have 20 "

relatively open wire mesh structures or thin me- where B.W. the bandwidth at half power in

metallic coatings that are optically transparent. Number of waves per centimeter of the pass band

are, for example, a coating of tin oxide des, L is the interaction length of the optical and

a few thousand angstroms thick. 'acoustic fields in the crystal and J η the dop-

At the electrodes 3 and 4 there will be a tension of the crystal.

applied at radio frequency via conductors 5 and 6, which are connected to the light transmission percentage by a signal generator having an optical filter 1 at the optical bandpass frequency source impedance 8. A light source 9 is quenz / ₀ is shown in FIG. 2 shown. _{The percentage of the light of the optical bandpass frequency / 0} to be filtered so that it is arranged in such a way that it is diffracted by a linear input polarizer 12 to 30 from the input polarization into the cross polarization polarization of the light beam 11 in a first position is shown in curve 16 as a function of the linear direction, for example the vertical Rieh acoustic power density in the crystal 2, is projected. The polarized light beam arrives represents. The output beam 14 therefore has a transducer characteristic through the transparent electrode 3, which is reproduced by the curve 16 of the anisotropic birefringent crystal 2 at 35.

collinear diffraction at an acoustic wave to In the case of the piezoelectric quartz crystal 2 is

the polarization of this part of the incident Lieh- the quartz plate 2 preferably for a thickness thrust-

tes, whose optical frequency is based on the frequency of the type of vibration or thickness expansion vibration

acoustic wave is related by equation 1, cut mode to the highest possible

to diffract in light of a second polarization, the 40 factor Q for the acoustic resonance oscillation

is orthogonal to the direction of the first polarization. kind of preserve. Because the energy in the standing

The diffracted output light beam 11 then arrives at wave ß times the energy loss per cycle,

by a polarization analyzer 13, for example, a considerable increase in efficiency

a linear polarizer and is in the direction by the use of a resonance oscillation

of the second (e.g. horizontal) polarization 45 type with high Q can be expected. The required

polarized so that the output of the polarization acoustic frequency to a pass band in the

analyzer 13 emits a beam 14, the only optical spectrum for typical birefringent

Generating materials from that part of the light of the input beam is on the order of magnitude

11 consists of the first to the second polarization from 10 to 100 MHz. When the crystal hits a

tion has been implemented. 50 fundamental wave resonance is tailored, so he will

The acousto-optical light filter J of FIG. 1 extremely thin in the case of quartz. Consequently, one power of the collinear acousto-optic utilization, a considerable increase in the required acoustics in an optically birefringent medium, is power for a transmittance of use. A crystal orientation is required for the 100 ⁰ O at the bandpass frequency, since the crystal 2 is chosen so that the incident linearly polar acoustic power density at 100 ⁰ O transmission-risen light beam 11 arrives at the acoustic wave with a capacity of 1 L-. Therefore, the operation of the input polarization in a second orthogonal at a higher harmonic order is preferred, whether polarization is bent. For a given aku gHch the coupling coefficient is reduced,
Each of the three thickness waveforms gives light frequencies a condition for the 60 odd harmonics, each of which torenJE and is cumulatively inflected. If the acoustic can be generated to change a passband in the static frequency, the optical spectrum to generate changes that of the assigned band of light frequencies, which is the acoustic acoustic frequency for each of the thickness optical elements from the first Polarization corresponds to the waveforms of the overtones. Hence second polarization diffracts. The diffraction in the second 65 can be a comb of optical frequencies, the near orthogonal polarization occurs in quartz over the roughly odd integer multiple of the photoelastic constant P ₁₄ and is only horizontal, simultaneously or separated by the cumulative, if the equation optical filter element sent by selective

5 6

tively the piezoelectrically caused resonance states can be reached via suitable gates on lines 5 or 6. For example, a quartz that can be connected in parallel can be y-cut around one or more. for the thickness thrust oscillation of the filter elements, select the shape of the circuit and a thickness of about 1.5 mm. to be connected to the resonator in order to be operated with the 9th .. 11th and 13th OK-ittons, ί> agreed desired overtone resonance to select different colors or optical frequencies. In FIG. 3, another acousto-optical filter, for example red, yellow and blue, is shown in the visible spectrum. The light filter 21 is to be emitted similar to the strand. In another example, an acousto-optical filter in the article cited quartz with a thickness of 1,398 mm with 11th .. 13th "Applied Physics Letters", vol Bandpass November 1969. The acousto-optical element is essentially the same in terms of the frequencies of the colors red, green and blue as that of FIG. 1 with the wavelengths 6465, 5471 and 4183 A. Exception that the electrode 4 reflects, so that the width of the bandpass between the points at the incident light beam, through a Rochon-Polahalber power is for such a crystal risationspristna 22 vertically polarized, after that at a pin wavelength of 5893 A (corresponding to the ls passage by the resonator 2 is reflected yellow optical frequency) about 272 A. There is and you diffracted output beam 11 in the prism 22 is therefore in the above example that this is reflected. The prism 22 is arranged step-by-step with the acoustic-optical filter in such a way that light, which can be fed with respect to the frequency that is cross-polarized to the input light, corresponds to the output beam 14 13th and 17th overtones, Thus, the output beam 14 has the or simultaneously red, green and blue colors as the bandpass characteristic of the curve 16 of Fig. 2. Output beam 14. By setting the amplitude in Fig of the oscillations of the overtone crystal 2 as an example of the invention. The acoustically the amount of red, green and blue regulated optical light filter 23 of FIG. 4 is essentially be. The three overtones can be excited simultaneously or step-by-step as described previously with reference to FIG. 1, but the analyzer has the desired color output by mixing Green and blue replaced during polarization analyzer or prism 24. the transmission period. The prism 24 is arranged in such a way that light of the same polarization as the input light beam II for emitting a specific fame is passed through the Rochon prism 24 as an output beam 25 with an optical transmission Light of the incoming polarization are capable of at the respective wavelength. Preferred orthogonal polarization of the prism 24 as wise with a large piezoelectric constant output beam! 14 is reflected. The output and a large acousto-optic constant and beam 14 has chosen the bandpass characteristic of the curve of a small birefringence. The acu 16 of Fig. 2. whereas the Aiisg.ingsMrahl 25 the static frequency is determined by equation (1). Transmission characteristic of curve 26 in FIG. 2 The remaining parameter is the order of the optical bandpass frequency. Hence overtonecs. This parameter is selected in order to obtain the desired bandwidth for the filter, generated by the frequency generator 7 acoustic frequencies. 40 zen f _R. / ,, and I _n . which the optical bandpass filter if a large bandwidth is desired for the filter / "is in the red, green and blue spectrum. a relatively small thickness is chosen, and they correspond. The output spectrum for the acoustic frequency is generated by a piezoelectric beam 14 which has the spectral characteristics of the excited resonance at an overtone of a relatively spectral diagram (a). whereas the low order output beam was chosen, the frequency of which corresponds to the 45 25 spectral diagram (b). The desired acoustic frequency is tuned. In Fig. 5, a ring laser 41 is shown. The opmit is the thickness in terms of the bandwidth tables resonator for the optical maser 41 wi, d determined and limited to a desired subfrequency by three mirrors 42, 43 and 44, which provide in the (submultiple). If a narrow corner of a triangle is sought to reflect the optical bandwidth, the resonator 5 ° beam of the optical maser 41 would be made in a closed, relatively thick manner and the overtone order would be arranged in a loop. The output mirror! increases. 42 is partially reflective, so that a relatively small amount of the acousto-optical light filter 1, the ncr portion of the incident light as an off-Fig. 1, the acoustic resonator or vibration beam 45 can pass through. A Paai tor 2 at its acoustic resonance frequency is over 55 acousto-optical filer. a frequency can be operated as described in reference to FIG. 1 in the light path of the opti-Sicnalgencrator 7 is fed to it. On the other hand see resonator may be disposed and on which the acoustic resonator 2 is similar tuned as a frequency-determining bandpass frequency and erwendet with the gleichet element of the driver circuit \ and Errcgungsfrequenz excited obtained from the frequency Gene frequency or signal genes. ^ IIOR 7 by a ^one-6o rator 7 will. The filters 1 will be replaced by amplifiers with positive feedback, wisely with sufficient acoustic power density ii, the frequency-determining crystal 2. operated by the acousto-optical device, so that in this case the filter oscillates only at a transmission capacity of the acoustic oscillation frequency of the crystal resonator 100 "0 at the bandpass frequency Bandpass frequency of the acousto-optical: electrical filter elements are used to select the filter 1 with the desired optical laser, the special overtone frequency to be excited

7 8

optical filters are excited to different In such a case, the light beam can be either

Obtain colored light outputs for beam 45. for phase velocity or for group

In Fig. 6, an optical maser 46 is shown. speed with corresponding advantages and

The optical maser 46 has an optical reso disadvantage of being collinear.

nator, which is limited by the distance between the mirror 5 when the light beam with the phase velocity 47 and the output mirror 48. Since the signal is collinear, the advantage of the cos-dependent mirror 48 is only partially reflective, so that the bandwidth center frequency is obtained with a relatively small proportion of about one percent of the deviation with the disadvantage that the incident light is used as the output beam 49 beam quickly migrates out of the acoustic beam. A gain medium 51 and an io since the energy propagates in the direction of the group acousto-optical filters 1 of the speed referenced and not of the type described in FIG. 1 in which the optical beam is collinear. Therefore, a narrow band resonator between the mirrors 47 and 48 can be characteristic in this case at the expense of either arranging it. The optical maser 46 can be achieved over a relatively higher acoustic aperture or a higher-wide range of optical output frequencies 15 ren acoustic power.
If, on the other hand, the light beam is made collinear to the excited overtones of the crystal 2 in the acu group velocity, the table optical filter 1 becomes tunable. In Fig. 7 the advantage of the effective use of the acoustics is shown an output where the gain see energy, but the angle between the maser as a function of the reciprocal of the optical 20 the acoustic vector K and the optical Vek wavelength of the output beam 49 shown - gate K is no longer zero and the shift im is set. A ₁ , A ₃ , A ₅ , A ₇ and A "correspond to different centers of the optical passband with an odd overtone η (η odd, integer deviation in the optical beam is a function of lig) of the used in the acoustic-optical filter. of the cos of the angle between the acoustic and th crystal 2. The output beam is directed to this optical vector K, which is no longer zero. This different output frequencies are matched by leading to a wider band pass at the same angular deviation fed to the corresponding overtone of the crystal. In borderline cases, in which the propagation is neither exactly collinear to the

The term »light« means in this registration phase velocity or group velocity

generation of electromagnetic radiation. This kind of light will overlap accordingly

does not need to be limited to the visible spectrum. With such birefringent crystals, at

to be. The merits of collinear diffraction are: phase velocity and group velocity are not collinear, the non-

1. The narrow angular tolerance characteristic of collinear propagation of the light in the filter to the Brazilian diffraction is mitigated. A 35 similar adverse effects for the filter. angular deviation of the incident of the acoustic power and the acoustic vector K and the incident 40 with the required bandpass filtering character optical vectors K; to tolerate the risks.

As can be seen from equation 2, the bandwidth is

2. the extent of the interaction between the acoustic and acoustic-optical filters is inversely dependent see and optical rays is because of the length L of the interaction through the collinear propagation intensified; therefore can 45 crystal. With acoustic-optical resonance filters The efficiency of the light conversion can be relatively short and therefore the band almost too long 100% increased and the required width be relatively wide. In such cases, the acoustic power for effective um- vectors for optical and acoustic waves Settlement can be drastically reduced. show a considerable discrepancy while

50 still takes advantage of collinear diffraction

In some birefringent crystals, since the acoustic-optical alternating vector of the phase velocity and the group effect occurs at a relatively high factor Q , the velocity is not collinear. The angle between and the interaction only with the cos des Winßinen can be about 20 ° in the case of quartz. kels of the deviation falls.

1 sheet of drawings

Claims (3)

1 2 'qufcnz the acoustic wave from 1050 to 750 MHz Patent claims: was changed. Such a tunable acoustic-optical filter is in the article »Elek- 1. Acoustic-optical filter with an in tronicaUy Tunable Acousto-Optic Filter «in the time direction on a downstream analyzer 5 “Applied Physics Letters”, Vol. 15, No. 10, translucent optically anisotropic medium, in 1969, pp. 325 and 326, described. which by means of an electro-acoustic, with such acoustic-optical devices An RF signal fed acoustic transducer was the acoustic wave in the birefringent Waves collinear with the direction of the light crystal by means of an electro-acoustic transducer Beam path can be generated through the medium, io excited, which is attached to one end of the crystal characterized in that the op- was. At the end of the crystal at the interface table anisotropic medium (2) piezoelectric between the crystal and the electro-acoustic Has properties and in connection with a converter an optically reflective coating Electration that can be acted upon by the HF signal is provided. The light to be filtered then arrives den assembly (3, 4) the electroacoustic 15 through the crystal in the first of the coating Converter forms. reflected direction and left the crystal in the 2. Filter according to claim 1, characterized in the opposite direction. While this arrangement draws, that at least one of the two electrons is suitable for certain filter applications, den (3, 4) made transparent and there is a need for an acousto-optical of the light beam 20 incident in the medium (2) is a resonance element in which the light to be filtered (11) is arranged. through the acousto-optical device 3. Filter according to claim 1 or 2, thereby passed. indicates that the medium (2) is used for a Reso- In another acoustic-optical filter nance of the acoustic wave is measured. the electro-acoustic converter on the side of the